Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of a display screen or a portion thereof with a transitional graphical user interface; and,

FIG. 2 is a front view of a display screen or a portion thereof with a graphical user interface transitioned in sequence from FIG. 1 .

The outer broken-line rectangle shows a display screen or a portion thereof and forms no part of the claimed design. The broken-line text and numbers illustrate portions of a graphical user interface and form no part of the claimed design.

The appearance of the images sequentially transitions between images shown in FIGS. 1 and 2 . The process or period in which one image changes to another forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or a portion thereof with a transitional graphical user interface, as shown and described. 